I don't think ebooks make a strong case for torrents mainly because of their small size and the sheer number.
Emule or IRC have been traditionally strong channels for ebook sharing.
It seems that, from what I've seen, when the media being torrented is small & numerous large collections are made - for example 1000s of SNES games.
You've ignored Rapidshare (and other clones)
Last time I used Rapidshare the free account had a restriction of something like 1 file per day. I wonder if it's still the case.
I am definitely unable to read more than one book per day so that would be plenty for me.
oh, it exists.
its not a torrent site, but http://a.aaaarg.org/ has a bunch of continental philosophy/economics pdfs (some are scans and some are ebooks).